The mobile Internet Protocol (IP) enables a mobile node to move freely from one point of connection to another in different networks which are visited along its route. More specifically, the mobile IP protocol describes those actions that enable a mobile node to maintain connectivity during a handover from one access router to another access router.
Mobile IP is described in specifications that may be found from the Internet Engineering Task Force (IETF), and the like, as for instance in RFC (Request For Comments) 3344 and RFC 3776. Mobile IP enables the mobile node to be identified by its home address, regardless of its current point of attachment to the Internet. When the mobile node is away from the home network, it is also associated with a care-of-address, which provides information about the mobile node's current location. Typically, during a handover between access routers the care-of-address changes but the home address remains the same.
The following provides a rough outline of operation of the state of the art mobile IP protocol:                Mobility agents (i.e., foreign agents and home agents) advertise their presence via Agent Advertisement messages. A mobile node may optionally solicit an Agent Advertisement message from any locally attached mobility agents through an Agent Solicitation message.        A mobile node receives these Agent Advertisements and determines whether it is on its home network or a foreign network.        When the mobile node detects that it is located on its home network, it operates without mobility services. If returning to its home network from being registered elsewhere, the mobile node deregisters with its home agent, through exchange of a Registration Request and Registration Reply message with it.        When a mobile node detects that it has moved to a foreign network, it obtains a care-of address on the foreign network.        The mobile node operating away from home then registers its new care-of address with its home agent through exchange of a Registration Request and Registration Reply message with it, possibly via a foreign agent.        Datagrams sent to the mobile node's home address are intercepted by its home agent, tunneled by the home agent to the mobile node's care-of address, received at the tunnel endpoint, and finally delivered to the mobile node.        In the reverse direction, datagrams sent by the mobile node are generally delivered to their destination using standard IP routing mechanisms, not necessarily passing through the home agent.        
When away from home, Mobile IP uses protocol tunneling to hide a mobile node's home address from intervening routers between its home network and its current location. The tunnel terminates at the mobile node's care-of address. The care-of address must be an address to which datagrams can be delivered via conventional IP routing. At the care-of address, the original datagram is removed from the tunnel and delivered to the mobile node.
The problem with this state of the art approach, is that the so called triangular routing is not data transport efficient, as all traffic to a mobile node in a foreign network is routed via the home agent in the home network of the mobile node. The use of the long term IP address belonging to the home network of the mobile node has made such a scheme necessary.